Low toxicity shot pellets

ABSTRACT

Low toxicity shot or pellets for shotgun cartridges or the like comprises finely divided metallic particles, preferably a mixture of finely divided molybdenum and tungsten particles in a polymer matrix. The resulting pellets have a high density and are much less prone to damage the barrels of guns from which they are fired than prior suggested alternatives to lead shot. If desired, friction between the pellets and gun barrels may be further reduced by incorporating a lubricant, such as molybdenum sulphide or graphite in the polymer matrix.

This invention relates to shot pellets and the like.

Many thousands of tonnes of lead pellets are scattered on the surface ofthe earth and embedded in trees and fences each year in the act of bird,clay pigeon and small game shooting for business or pleasure. It is nowrecognised that where this shot falls on wetlands it may be taken up bymany birds in the belief that it can become part of the normalcomplement of pebbles or gravel that performs an essential duty in thecrops of these birds. Unfortunately, the outcome can be that the birdssuffer from progressive lead poisoning which can result in their deathor, equally disturbing, their value as human food becomes very suspect.A further problem now recognised is that lead may be dissolved from thedeposited shot and enter into the structure of crops grown for humanfood. A similar problem with the gathering up of lead fishing weights byswans seems to have been resolved by the adoption of alternative heavymaterials for making the weights. Attempts to apply a similar solutionto the shot used in making shotgun cartridges have proved much moredifficult because of the stringent requirements imposed on the physicalproperties of the shot by the severe conditions that they are exposed toin the firing of the guns.

A key property of lead that makes it so successful as a shot material isits high density, 11.35 tonnes per m³, because the energy associatedwith the shot at the moment that it strikes its target relates to itsmass and its velocity as E=1/2 mv². Lead has a modest position in thelist of abundancies of the metallic elements at 10 parts per million andposes no problem of dwindling resource. Iron has been proposed as analternative and has found some use but its density is only 7.86 tonnesper m³ which means that it only has 69.25% of the striking energyprovided by lead shot of the same size. Iron shot also offers problemsbecause of its rigidity which can damage the bores of costly sportingguns and even be hazardous when used in certain types of guns because ofthe development of abnormally high and dangerous pressures. The softnessof lead allows it to negotiate safely the bores of guns which are chokedat the barrel end to modify the pattern of the flying shot. Steel shotcan also give problems by its tendency to corrode and this process canbind the loose shot in a typical cartridge into solid slugs which candamage the gun. It is also reported to give difficulties in timbergrowing areas where the shot is embedded in tree trunks and presents ahazard to power driven woodsaws. Shot made of highly elastic metals,such as steel, also poses a hazard to participants and onlookers becauseit is far more prone to ricochet from hard surfaces than malleable leadshot.

Bismuth has also been proposed as a shot material because it has adensity of 9.747 tonnes per m³ which is higher than iron at 7.68 tonnesper m³, but its abundance is much lower than lead 0.004 parts permillion and it is a secondary metallurgical material that is to say aby-product of the refining of other metals. This means that its sourceis precarious and the price, already high, could escalate if attemptswere made to adopt it generally. Bismuth also suffers from being a veryweak and brittle metal and can only be made into a useable shot if it isalloyed with expensive tin or toxic lead. There are also unresolvedquestions about the possible toxicity of bismuth when ingested byanimals.

It is an object of the present invention to provide an improved shotmaterial which avoids the above-noted disadvantages of lead and is alsofree of the cost penalty, brittleness and possible toxicity of bismuth.

According to one aspect of the invention, there is provided a shotformed of a material comprising finely divided metallic particles in apolymer matrix.

The material from which the shot is formed may comprise a mixture offinely divided particles of molybdenum and tungsten in a polymer matrix.

According to another aspect of the invention there is provided acartridge including propellant retained within a casing and shotretained in the casing, the shot being composed of a material comprisingfinely divided metallic particles in a polymer matrix.

According to yet another aspect of the invention, there is provided amethod of making shot, including mixing finely divided metal particlesand a molten polymer, or the fluid precursor of a polymer, and formingthe resulting mixture, before or after solidification of the polymer,into shot.

The metal molybdenum has a very favourable density of 10.22 tonnes perm³ and a reasonable abundance at 1.2 parts per million. The metaltungsten has a density of 19.3 tonnes per m³ and an abundance of 1.0parts per million. Both molybdenum and tungsten are primary metals fromtheir mineral sources. Both of these heavy metals are assured inresource terms because of the commercially successful metallurgicalprocesses in which they are highly developed in the making of specialalloys. They have an enviable reputation for low toxicity well supportedby studies reported in the scientific literature. Other dense metals canbe used, for example hafnium and/or tantalum, which have densities of13.0 tonnes per m³ and 16.65 tonnes per m³ respectively, and areresistant to atmospheric oxidation. However, the toxicology of thesemetals is less well investigated than that of molybdenum and tungstenand winning the metals hafnium and tantalum from their ores is currentcomplex and costly. Niobium or holmium could be used but these havedensities which are only slightly greater than that of iron. Accordinglymolybdenum and tungsten are the preferred metals. These are hard metalswith high melting points and the fabrication of shot from these metalsalone would be costly whilst the resulting shot would, like the steelshot referred to above, tend to damage gun barrels.

The invention proposes a form of composite shot in which powdered metalfor example a mixture of powdered molybdenum and tungsten, is bound intoa solid pellet by the use of polymeric materials. Preferably thepolymeric material is present in just sufficient quantity to fill, oralmost fill, the voids between the particles of the powdered metal suchthat the mix is close to the condition of close packing of spheres Whichmeans that about two thirds of the volume is metallic powder. Thus, at70% by volume in a binder matrix of unit density, molybdenum alone wouldgive a pellet of density about 7.51 tonnes per m³. If only 23% of themetal in the mix is replaced by metallic tungsten then a pellet ofdensity 8.42 tonnes per m³ is created which would have 13.63% morestriking energy than a steel pellet and yet would be compliant becauseof the nature of the polymeric binder.

It is further proposed to include, in the polymer/metal powder mix,minor amounts of a lubricant substance such as molybdenum sulphide orgraphite which would further improve the performance and minimise thewear of the gun barrels.

The polymeric binder or matrix may be either a thermoplastic or athermosetting polymer. Suitable thermoplastic polymers are, for examplepolystyrene, chlorosulphonated polyethylene, and ethylene vinyl acetatecopolymer. Suitable thermosetting polymers are, for example, epoxyresins, phenol formaldehyde resins, or melamine formaldehyde resins.

It will be appreciated that the invention is also applicable tocartridges having a single shot or ball, and to ammunition for rifles,pistols or other small arms. The term "shot" as used herein is intendedto cover bullets to be fired from such small arms, and the term"cartridge" as used herein is intended to cover the combination of acasing containing propellant, and a bullet, forming a "round" for suchsmall arms.

Embodiments of the invention are described below by way of example.

In each of the following examples, a calculated blend of polymer andmetal powders is formed into pellets of near spherical form by methodsfamiliar to experts in the processing of filled plastics compositions,these pellets being used as charges on shot gun cartridges.

EXAMPLE (1)

Commercially purchased pure molybdenum powder and tungsten powdershaving average particle sizes of 10 micrometers were blended in theratio of 56.92% by weight of molybdenum and 43.08% by weight of tungstenand this mixture further cold preblended with powdered polystyrene ofdensity 1 tonne per m³. This preblend of powders was then hot compoundedat 160° C. in a Banbury type plastic compounding machine and thedischarged mass broken into a coarse powder using a typical plasticsindustry sprue granulator. This coarse powder was moulded into 5 mmdiameter spheres by injecting the heated material into a two componentsteel mould and the resultant moulded near spherical pellets were foundto have the expected density of 9.5 tonnes per m³.

EXAMPLE (2)

A technical grade of powdered roasted molybdenite was reduced tometallic molybdenum by heating it to 1,000° C. in a stream of hydrogengas. The resultant coarse molybdenum powder with a particle sizeaveraging 45 micrometers was blended with commercially purchasedtungsten powder having average particle size of 10 micrometers in theratio of 43.08% by weight of tungsten and 56.92% by weight ofmolybdenum. This blend of powdered metals was then pre-blended withpowdered polystyrene of density 1 tonne per m³ and the cold pre-blendfluxed and mixed at 160° C. using a laboratory 2-roll mill working ateven speed. The mixture has a composition 20% by volume of tungsten, 50%by volume of molybdenum, and 30% by volume of polystyrene and, whenmoulded in steel moulds to near spherical pellets was found to have theexpected density of about 9.26 tonnes per m³.

EXAMPLE (3)

A blend of commercially purchased molybdenum and tungsten powdersprepared as in example (1) above was mixed cold with commerciallyavailable spray dried water soluble melamine formaldehyde resin powderto which 0.5% by weight of monochloroacetamide catalyst had been added.The powder blend of all four ingredients was then densified by fluxingit on a 2-roll laboratory mixing mill at 135° C. for a few minutes. Thesoft hide cut from the mill was mechanically powdered, after cooling,and the powder converted into tough spherical pellets 5 mm diameter bycompression moulding at 150° C. in a multicavity flash type steelmoulding tool. The pellets ejected from the moulding tool cavities had atrace of brittle flash around their equators which was readily removedby tumbling them in a rotating hexagonal wooden drum for a few minutes.The pellets were found to have the expected density of about 9.38 tonnesper m³.

EXAMPLE (4)

A blend of commercially available molybdenum and tungsten powders in theratio of equal parts by weight of molybdenum and tungsten was furtherblended cold with a low viscosity epoxy resin which itself incorporated10% by weight of amine catalyst. The metal powder and resin were used insuch amounts as to give a resin:metal volume ratio of 60:40 and theresultant stiff paste was forced through a plate drilled with 5 mmdiameter holes by means of a simple ram and cylinder arrangement. Theemerging strands were crumbled by brushing them from the perforatedplate as they emerged and the irregular fragments were rolled in arotating drum fitted with internal raised vanes which prevented the masssliding around the drum. This technique is well known to those skilledin the art of pelletising pharmaceutical materials or mineral powders.The rolling action rapidly converted the irregular fragments into nearperfect spheres and the motion was maintained until the chemicalprocesses triggered by the amine catalyst caused the epoxy resin toharden sufficiently to confer adequate strength to the pellets to enablethem to be transferred to trays which were loaded into an ovenmaintained at 100° C. where they remained until they were hardened bythe completion of the curing process of the epoxy resin. The resultingnear spherical pellets had diameters between 2 and 6 mm and densityclose to the calculated 8.87 tonnes per m³.

The pellets manufactured as described in any of Examples 1 to 4 abovemay be incorporated in a shotgun cartridge in which the propellant isretained within a casing by a wad above which a number of the nearspherical shot pellets are situated, the pellets being retained bycrimping the extremity of the casing or by some other readily releasableclosure means, such as a further wad, for example in the form of acardboard disc.

I claim:
 1. A shotgun cartridge comprising a plurality of individualshot pellets, each formed of a material comprising finely dividedparticles of metallic molybdenum in a polymer matrix.
 2. A cartridgeaccording to claim 1, wherein said shot pellets are compliant.
 3. Acartridge according to claim 1, wherein said pellets are formed of amaterial comprising a mixture of finely divided particles said ofmolybdenum and tungsten in said polymer matrix.
 4. A cartridge accordingto claim 3, wherein the metal component of said material comprises from40% to 60% molybdenum.
 5. A cartridge according to claim 1, wherein thematerial forming said pellets comprises 70% by volume of said metallicparticles.
 6. A cartridge according to claim 1, wherein said pelletshave a density of 8.87 to 9.5 tons/m³.
 7. A cartridge according to claim1, wherein said polymer matrix comprises ethylene vinyl acetatecopolymer.
 8. A cartridge according to claim 1 including, including insaid polymer matrix, a friction reducing substance selected from thegroup consisting of molybdenum sulphide and graphite.
 9. A shotguncartridge according to claim 1, wherein said polymer matrix comprises athermoplastic.
 10. A shotgun cartridge according to claim 9, whereinsaid thermoplastics comprises polystyrene.
 11. A shotgun cartridgeaccording to claim 11, wherein said thermoplastic comprisespolyethylene.
 12. A method of making pellets for shotgun cartridges,comprising providing a mixture comprising finely divided particles ofmetallic molybdenum in a plastic binder and converting the mixture intopellets by compression molding in a multi-cavity flash type moldingtool.
 13. The method of claim 12 includingincluding in said plasticsbinder a friction reduction substance.
 14. The method of claim 12,wherein said mixture comprises a mixture of finely divided particles oftungsten and said molybdenum in said polymer matrix.
 15. A method ofmaking pellets for shotgun cartridges comprisingproviding a mixturecomprising finely divided metallic particles in a plastic binder;converting the mixture into pellets by compression molding in amulti-cavity flash type molding tool; including in said plastics bindera friction reduction substance; and selecting said metalllic particlesfrom the group consisting of molybdenum and tungsten.
 16. A shotguncartridge comprising a plurality of individual shot pellets, each formedof a material comprising finely divided metallic particles in a matrixof ethylene vinyl acetate copolymer.
 17. A cartridge according to claim16, wherein said pellets are formed of a material comprising a mixtureof finely divided particles of tungsten and molybdenum in said matrix ofethylene vinyl acetate copolymer.